Supply-noise-rejecting current source

ABSTRACT

Various technologies pertaining to a high-impedance current source are described herein. The current source outputs a substantially constant current by way of a first transistor that draws current from a supply. The current source is configured to feed-back noise from the supply to a feedback resistor at an input of an operational amplifier (op-amp) by way of a second transistor. The feedback resistor and the op-amp are configured such that responsive to receiving the supply noise feedback, the op-amp drives a gate voltage of the first transistor to cause the first transistor to reject the supply noise and cause the output of the current source to remain substantially constant.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. application Ser. No.15/660,296, filed Jul. 26, 2017, which is incorporated herein byreference in its entirety.

STATEMENT OF GOVERNMENTAL INTEREST

This invention was made with Government support under Contract No.DE-NA0003525 awarded by the United States Department of Energy/NationalNuclear Security Administration. The Government has certain rights inthe invention.

BACKGROUND

Current sources provide a fixed current, or a current dependent on someknown control input (e.g., a control voltage input). An ideal currentsource has infinite impedance looking into either of its two terminals.Conventional practical current sources realized with transistorsgenerally have asymmetric impedance, wherein an impedance looking into afirst terminal of the current source can be orders of magnitude greaterthan an impedance looking into a second terminal. In some applications,current sources require sourcing current from positive or negativevoltage supply rails. However, voltage supply rails are inherently noisybecause they are generated by active circuits. While a conventionalcurrent source may have a high impedance looking into an output terminalof the conventional current source, the asymmetric impedance of theconventional current source causes the output current of theconventional current source to be poorly isolated from voltage noise onthe supply rails. Voltage supply noise transferred to the output of aconventional current source can have a magnitude several times greaterthan the intrinsic electronics noise of the current source itself andcan therefore dominate the noise performance of a given electronicsystem. Thus, conventional current sources may be unsuitable for someapplications requiring very low-noise operation due to feedthrough powersupply noise.

SUMMARY

The following is a brief summary of subject matter that is described ingreater detail herein. This summary is not intended to be limiting as tothe scope of the claims.

A supply-noise-rejecting current source is described herein, wherein thecurrent source has a substantially symmetric impedance at both drivelevel terminals of the output device, e.g., drain and source. Thecurrent source uses a feedback technique to apply the noise on therespective power supply to the gate (or base for a bipolar transistor)of the current source output transistor, over a controllable bandwidth.This action, independent of the transistor current or added sourceimpedance, rejects supply noise feeding through to the current-sourceoutput, the drain. The current source outputs a substantially constantcurrent by way of a first output transistor that draws current from asupply. The current source is configured to feed-back noise from thesupply to a feedback resistor at an input of an operational amplifier(op-amp) by way of a second transistor. The op-amp and the secondtransistor are configured such that responsive to receiving the supplynoise feedback, the op-amp drives a gate voltage of the first transistorto cause the output of the current source to remain substantiallyconstant. In an exemplary embodiment, the feedback resistor is connectedto a first input terminal of the op-amp and a ground reference node, anda control voltage is connected to a second input terminal of the op-amp.A drain of the second transistor is connected to the feedback resistorand the first input of the op-amp, a gate of the second transistor isconnected to an output terminal of the op-amp and the gate of the firsttransistor, while a source of the second transistor is connected to thesupply. Noise on a voltage supply induces a current from the source tothe drain of the second transistor causing a change in voltage at thefirst input of the op-amp via the feedback resistor. Responsive to thechange in voltage at the first op-amp input, the op-amp drives itsoutput to equalize the voltage at the first op-amp input to the secondop-amp input. The op-amp thus outputs a voltage at the gate of the firstand second transistors that is approximately equal to the noise voltagegenerated by the supply. In turn, this causes the first transistor toreject noise output by the supply.

In another exemplary embodiment, a current mirror can be configured toincrease an output impedance of the current source at the outputterminal of the current source by feeding back stimulation received atthe output terminal to the feedback resistor connected to the firstinput of the op-amp. When the output stimulation is fed back to theresistor at the first input of the op-amp, the op-amp drives its outputvoltage at the gate of the output transistor to maintain the outputcurrent to a substantially constant level.

In another embodiment, a third transistor forms a current mirror withthe second transistor. The current mirror is configured to increase anoutput impedance of the current source at the output terminal of thecurrent source. A source of the second transistor is connected to thesupply, and a drain of the second transistor is connected to the inputof the op-amp. A source of the third transistor is connected to thesupply, a gate of the third transistor is connected to a gate of thesecond transistor, and a drain of the third transistor is connected tothe source of the first transistor. In this embodiment, a fourthtransistor cascodes the second transistor to form a symmetricalvoltage-balanced current mirror.

The above summary presents a simplified summary in order to provide abasic understanding of some aspects of the systems and/or methodsdiscussed herein. This summary is not an extensive overview of thesystems and/or methods discussed herein. It is not intended to identifykey/critical elements or to delineate the scope of such systems and/ormethods. Its sole purpose is to present some concepts in a simplifiedform as a prelude to the more detailed description that is presentedlater.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of an exemplary supply-noise-rejectingcurrent source.

FIG. 2 is a circuit diagram of another exemplary supply-noise-rejectingcurrent source.

FIG. 3 is a circuit diagram of yet another exemplarysupply-noise-rejecting current source.

FIG. 4 is a circuit diagram of still yet another exemplarysupply-noise-rejecting current source.

FIG. 5 is a block circuit diagram illustrating an exemplary photodiodeamplifier application for embodiments of the supply-noise-rejectingcurrent source.

DETAILED DESCRIPTION

Various technologies pertaining to a current source with a high andrelatively symmetric input and output impedance are now described withreference to the drawings, wherein like reference numerals are used torefer to like elements throughout. In the following description, forpurposes of explanation, numerous specific details are set forth inorder to provide a thorough understanding of one or more aspects. It maybe evident, however, that such aspect(s) may be practiced without thesespecific details. In other instances, well-known structures and devicesare shown in block diagram form in order to facilitate describing one ormore aspects. Further, it is to be understood that functionality that isdescribed as being carried out by certain system components may beperformed by multiple components. Similarly, for instance, a componentmay be configured to perform functionality that is described as beingcarried out by multiple components.

Moreover, the term “or” is intended to mean an inclusive “or” ratherthan an exclusive “or.” That is, unless specified otherwise, or clearfrom the context, the phrase “X employs A or B” is intended to mean anyof the natural inclusive permutations. That is, the phrase “X employs Aor B” is satisfied by any of the following instances: X employs A; Xemploys B; or X employs both A and B. In addition, the articles “a” and“an” as used in this application and the appended claims shouldgenerally be construed to mean “one or more” unless specified otherwiseor clear from the context to be directed to a singular form.Additionally, as used herein, the term “exemplary” is intended to meanserving as an illustration or example of something, and is not intendedto indicate a preference.

With reference to FIG. 1, an exemplary system 100 that functions as asupply-noise-rejecting current source is illustrated. The system 100includes a first voltage supply V11 and a second voltage supply V12connected to a common ground reference node GND. In exemplaryembodiments, the voltage supplies V11, V12 are positive and negativesupply rails of a voltage supply unit (not shown). The system 100further comprises an operational amplifier (op-amp) U11. In an example,the op-amp U11 comprises an operational transconductance amplifier(OTA). For the purposes of description of the exemplary systemsdescribed herein, op-amps will be described as having characteristics ofideal op-amps (i.e., infinite impedance at each input terminal, infinitegain and identical voltage at each input terminal), however it is to berecognized that practical op-amps exhibit some non-ideality. The OTAdiffers from a generic op-amp in that it does not have a low outputimpedance final stage. The op-amp U11 is connected to the first voltagesupply V11 at a first supply terminal 102 of the op-amp U11. The op-ampU11 is further connected to the second voltage supply V12 at a secondsupply terminal 104 of the op-amp U11. The system 100 further includes aresistor R11 that is connected to a first input terminal 106 of theop-amp U11 and a control voltage source V13 that is connected to asecond input terminal 108 of the op-amp U11. The system also comprises afirst transistor M11 and a second transistor M12, shown as NMOStransistors. In the exemplary system 100, a drain 110 of the firsttransistor M11 is an output terminal of the current source, sourcing acurrent I_(out). A source 112 of the first transistor M11 is connectedto the second voltage supply V12, and a gate 114 of the first transistorM11 is connected to an output 116 of the op-amp U11. A drain 118 of thesecond transistor M12 is connected to the resistor R11 and the firstinput 106 of the op-amp U11. A source 120 of the second transistor M12is connected to the second voltage supply V12, and a gate 122 of thesecond transistor M12 is connected to the output 116 of the op-amp U11and the gate 114 of the first transistor M11.

Operation of the exemplary system 100 is now described. The system 100is configured to drive a voltage V_(g) on the gate 114 of the firsttransistor M11 to match supply noise from the second voltage supply V12that sources current to the first transistor M11. By way of example,voltage V_(g) of system 100 can have an amplitude and a phaseapproximately equal to supply noise from the current-sourcing voltagesupply V12. The second voltage supply V12 can be configured to have a DCor AC voltage characteristic. For example, the voltage supply V12 can beconfigured to have a constant DC voltage of −5V from a negative terminal124 to a positive terminal 126 of the supply V12 (the positive terminal126 connected to the ground reference node GND). In addition to theconfigured output of the supply V12, the supply outputs some noise thatis ordinarily a small fraction of the configured output. By way ofexample, if the supply V12 is configured to have a DC voltage output of−5V_(g), the supply V12 can output noise having an RMS value ofapproximately 30 μN. Voltage supply noise is generally low frequency(e.g., 10 Hz to 10 kHz) and 1/f in nature. The first transistor M11outputs a substantially constant current I_(out) (e.g., a constant DCcurrent±0.1%, a time-varying AC current±0.1% RMS), a value of whichdepends upon a resistance value of the resistor R11 and a voltage outputof the voltage source V13.

The second transistor M12 is configured to feed-back supply noise fromthe supply V12 to the resistor R11. When voltage supply noise occurs onthe voltage supply V12, a current flows through the source 120 and drain118 of the second transistor M12. Since current does not flow into theinput terminals of the op-amp U11, the current that flows through thesecond transistor M12 also flows through the resistor R11, yielding avoltage change at V_(R11), the voltage across the resistor. The op-ampU11 outputs a voltage V_(g) at its output terminal 116, which isapproximately equal to the supply noise voltage output by V12. Thevoltage V_(g) causes the current through the transistor M12 to change sothat the voltage V_(R11) is equal to the control voltage V13, thereforethe output current, I_(out) will equal V13/R11 for this example. Thevoltage V_(g) is applied to the gate 114 of the first transistor M11 andthe gate 122 of the second transistor M12. Since the voltage V_(g) isapproximately equal to the supply noise voltage output by V12 inamplitude and phase, the gate-to-source voltage of each of thetransistors M11, M12 with respect to the time-varying supply noisevoltage is approximately zero. Thus, the impedance of the transistor M11with respect to the supply voltage noise from V12 is very high (e.g., onthe order of 100 kΩ or MΩ), and the transistor M11 rejects the supplynoise. Therefore, feeding back the supply noise from the supply V12 tothe resistor R11 by way of the second transistor M12 causes the outputvoltage V_(g) at the output 116 of the op-amp U11 approximately equalthe supply noise, and causes the first transistor M11 to reject thesupply noise from V12. The system 100 therefore functions as a symmetricimpedance current source, since the transistor M11 has a high impedance(e.g., 100's kΩ to MΩ's) looking into its output terminal, the drain110, and a high impedance (e.g., 100's kΩ to MΩ's) relative to thesupply V12.

Referring now to FIG. 2, an exemplary system 200 functioning as asupply-noise-rejecting current source is illustrated wherein the system200 is tuned for various operating parameters. The system 200 includesthe voltage supply sources V11 and V12 connected to the ground referencenode GND, the op-amp U11, the control voltage source V13, the first andsecond transistors M11, M12, and the feedback resistor R11, connected toone another as in the system 100.

The system 200 additionally includes resistors R21, R22, R23 and acapacitor C21. The resistor R21 and the capacitor C21 are connected inseries, wherein the resistor R21 is connected to the output terminal 116of the op-amp U11 and the gates 114, 122 of the first transistor M11 andthe second transistor M12, respectively. The capacitor C21 is connectedbetween the resistor R21 and ground. The resistor R21 and the capacitorC21 are chosen to select an operational bandwidth and noise-bandwidth ofthe feedback loop from the output 116 of the op-amp U11 to the firstinput 106 of the op-amp U11. Below the bandwidth frequency, the system100 drives the voltage V_(g) to approximately equal the supply noisefrom the supply V12, and therefore the first transistor M11 has a highsymmetric impedance from the output I_(out) and from the voltage supplyV12, as described above with respect to FIG. 1. The resistor R22 isconnected between the source 112 of the first transistor M11 and thenegative terminal 124 of the voltage supply V12. The resistor R23 isconnected between the source 120 of the second transistor M12 and thenegative terminal 124 of the voltage supply V12. The resistors R22 andR23 can be included in the system 200 to reduce electronics noiseassociated with the transistors M11 and M12 and also increase thecurrent source impedances at output node 110. In particular, electronicsnoise associated with using discrete transistor components for thetransistors M11 and M12 can be reduced by including the resistors R22and R23. In integrated circuit implementations (e.g.,application-specific integrated circuits, or ASICS), electronics noisecan be reduced through control of physical specifications of thetransistors M11 and M12 (e.g., geometry of gates, sources, drains,etc.). Resistance values of the resistors R22 and R23 can beindependently selected based upon design considerations for a particularapplication of the current source 200.

Referring now to FIG. 3, an exemplary system 300 that functions as acurrent source is shown wherein a current mirror is used to increase anoutput impedance of the current source. The current mirror feeds backstimulation received at the output of the system 300 to a feedbackresistor connected to an op-amp, where the op-amp is configured tooutput the stimulation to the gate of an output transistor. The system300 comprises the voltage supplies V11, V22, connected to groundreference node GND and the op-amp U11 as in the systems 100, 200; theresistor R11 connected to the first input terminal 106 of the op-ampU11; the control voltage source V13 connected to the second inputterminal 108 of the op-amp U11; and the resistor R21 and capacitor C21connected between the output terminal 116 of the op-amp U11 and theground reference node GND.

The exemplary system 300 further comprises a first output transistor M31wherein a gate 302 of the first transistor M31 is connected to theoutput terminal 116 of the op-amp U11. The drain 304 of the firsttransistor M31 functions as an output terminal of the current source,while a source 306 of the first transistor M31 is connected to a currentmirror 308. As in the exemplary system 100, the first transistor M31outputs a substantially constant current I_(out) that depends upon aresistance of the resistor R11 and a voltage of the voltage source V13.The current mirror 308 is configured to increase an impedance of thecurrent source 300 looking into the output terminal at the drain 304 ofthe first transistor M31 by feeding back stimulation received at thedrain 304 of the first transistor M31 to the resistor R11. For example,feeding back the output stimulation to the first input terminal 106 ofthe op-amp U11 by way of the current mirror 308 can increase the outputimpedance of the current source looking into the output terminal 304from an impedance on the order of MΩ's to many 10's of MΩ.

The current mirror 308 comprises a second transistor M32, a thirdtransistor M33, and a fourth transistor M34. The gate of the secondtransistor M32, 314 is connected to the gate and the drain of thirdtransistor M33, 318 and 316 respectively. The sources of transistors M32and M33, nodes 312 and 320 respectively are connected to the supplyvoltage, V22, node 124. The fourth transistor M34 cascodes the secondtransistor M32 via the gate of the first transistor M31, 302. Thisconfiguration makes second transistor M32 and third transistor M33function as a current mirror cascoded by fourth transistor M34. Fourthtransistor M34 cascodes the drain of second transistor M32 and providesthe feedback current to resistor R11. With second transistor M32 matchedto third transistor M33 and first transistor M31 matched to fourthtransistor M34, the open loop impedances at transistor sources 312 and320 are equal and symmetrical, this allows very high supply noiserejection in this closed loop circuit. The fourth transistor M34 istherefore configured to feed-back supply noise induced current from thesupply V22, 124, to the feedback resistor R11 via a drain, 310,whereupon the op-amp U11 outputs a signal to the gate of M31, node 302,that forces the drain current of M31 to be independent of the ac voltage(noise) from V22 on node 124.

The feedback loop of this circuit forces the voltage at R11, node 310 tobe substantially constant thereby rejecting current being induced intothe drain of M31 via node 304 or the voltage supply node 124. Therefore,the current mirror 308 is configured to increase an output impedancelooking into the drain 304 of the first transistor M31 via the samefeedback circuit that rejects power supply noise, node 124, fromstimulating current to the drain of M31, I_(out).

Referring now to FIG. 4, still another exemplary supply-noise-rejectingcurrent source 400 is shown. This circuit operates substantially thesame to the above circuit shown in FIG. 3, but adds resistors to thesources of transistor M32 and M33 to allow additional designflexibility. The current source 400 comprises the ground reference nodeGND, the voltage sources V11, V22, the control voltage source V13, theop-amp U11, the feedback resistor R11, the first transistor M31, theresistor R21 and the capacitor C21, connected as described above withrespect to FIG. 3. The current source 400 further comprises a resistorR41 that is connected between the source 312 of the second transistorM32 such that the source 312 of the second transistor M32 is connectedto the negative terminal 124 of the voltage supply V22 through theresistor R41. The current source 400 also includes a resistor R42 thatis connected between the source 320 of the third transistor M33 suchthat the source 320 of the third transistor is connected to the negativeterminal 124 of the voltage supply V22 through the resistor R42. Thefourth transistor M34 is therefore configured to feed-back supply noiseinduced current from the supply V2, 124, to the feedback resistor R11via a drain, 310, whereupon the op-amp U11 is connected to V_(g), thegate 302 of the first transistor M31. The resistors R42 and R41 can beused to reduce electronics noise associated with the transistors M31-M33and M34, respectively, while increasing the output impedance of M31.

Referring now to FIG. 5, an exemplary application of asupply-noise-rejecting current source in a photodiode amplifier circuit500 is illustrated. The photodiode amplifier circuit 500 comprises apositive voltage source V51 with its negative terminal connected toground and a negative voltage source V52 with its positive terminalconnected to ground. The positive voltage source V51 and the negativevoltage source V52 each exhibit some supply noise modeled as the voltagesources 502 and 504, respectively. The circuit 500 further comprises aphotodiode D51 connected to the positive voltage source V51 (shown inFIG. 5 with the photodiode D51 connected to the modeled noise source502) and to a low-noise amplifier U51. As shown in FIG. 5, the low-noiseamplifier U51 is configured as a transimpedance amplifier where the gainis controlled by a resistor R51 and capacitor C51, the resistor R51 andthe capacitor C51 connected in parallel between an output terminal 506of the low-noise amplifier U51 and an inverting input terminal 507 ofthe low-noise amplifier U51. The circuit 500 additionally includes asupply-noise-rejecting voltage-controlled current source I51 that isconnected at a first terminal to the diode D51 and the low-noiseamplifier U51 and is connected at a second terminal to the negativevoltage source V52 (shown in FIG. 5 with the current source I51connected to the modeled noise source 504). The current source I51 iscontrolled by a voltage output V_(C) of a control loop 508 connected tothe low-noise amplifier U51. The control loop 508 is configured to setthe current output of the current source I51 equal to the low frequencybackground current of the photodiode D51 by feedback of the output ofthe low-noise amplifier U51 to the source I51. With the output of thecurrent source I51 controlled to equal the low frequency backgroundcurrent of the photodiode D51, higher frequency current in thephotodiode D51 will be amplified by the low noise amplifier U51. Thesupply-noise-rejecting current source I51 has high impedance lookinginto both of its terminals, and thus the current source I51 does notpass noise from the negative voltage source V52 onto the node V_(D) thatconnects the photodiode D51 and the current source I51, and thereforewill not be amplified by the low noise amplifier U51 to V_(OUT). If I51were replaced with a conventional current source, the relatively lowimpedance of the conventional current source looking into the terminalconnected to the negative voltage supply V52 would allow noise to bepassed onto the node V_(D), which would be amplified by the amplifierU51 and therefore passed with gain to the output V_(OUT) of thephotodiode amplifier circuit 500. The supply-noise-rejecting currentsource I51 is therefore useful in certain applications where therelatively low impedance of one side of a conventional current sourcerenders the conventional current source unsuitable.

It will be apparent to one of skill in the art that while thetransistors are shown as NMOS transistors in the exemplary systems 100,200, 300, and 400 described herein, the systems 100, 200, 300, and 400and other systems can be implemented in accordance with the technologiesdescribed herein using other types of transistors. For example, withminor modifications the systems 100, 200, 300, 400 can be implementedwith N or P type transistors, MOS, JFETs, BJTs, etc., depending on thedesired polarity of the current source output.

What has been described above includes examples of one or moreembodiments. It is, of course, not possible to describe everyconceivable modification and alteration of the above devices ormethodologies for purposes of describing the aforementioned aspects, butone of ordinary skill in the art can recognize that many furthermodifications and permutations of various aspects are possible.Accordingly, the described aspects are intended to embrace all suchalterations, modifications, and variations that fall within the spiritand scope of the appended claims. Furthermore, to the extent that theterm “includes” is used in either the details description or the claims,such term is intended to be inclusive in a manner similar to the term“comprising” as “comprising” is interpreted when employed as atransitional word in a claim.

What is claimed is:
 1. A current source configured to output asubstantially constant current, comprising: an operational amplifier(op-amp); a feedback resistor connected to an input of the op-amp and aground reference node; a first transistor that sources current from asupply, wherein a source or a drain of the first transistor is an outputterminal of the current source; a second transistor configured tofeed-back supply noise to the feedback resistor, wherein responsive tothe second transistor feeding back the supply noise to the feedbackresistor, the op-amp outputs the supply noise to a gate of the firsttransistor; an operational bandwidth network, the operational bandwidthnetwork including: a second resistor; and a capacitor connected inseries with the second resistor, a first terminal of the operationalbandwidth network connected to the output of the op-amp, wherein anoperational bandwidth of the current source is based upon a resistancevalue of the second resistor and a capacitance value of the capacitor;wherein a second input of the op-amp is connected to a control voltagesource, the control voltage source further connected to the groundreference node; and wherein the substantially constant current dependsupon a resistance value of the feedback resistor and a voltage output ofthe control voltage source.
 2. The current source of claim 1, whereinthe current source is a voltage-controlled current source.
 3. Thecurrent source of claim 1, further comprising a third transistor, thethird transistor forming a current mirror with the second transistor,the current mirror configured to feed back a stimulation received at theoutput terminal of the current source to the feedback resistor.
 4. Thecurrent source of claim 1, further comprising a third transistor, thethird transistor forming a current mirror with the second transistor,the current mirror configured to increase an output impedance of thecurrent source at the output terminal of the current source.
 5. Thecurrent source of claim 4, wherein a source of the second transistor isconnected directly or through a third resistor to the supply, whereinfurther a source of the third transistor is connected directly orthrough a fourth resistor to the supply, a gate of the third transistoris connected to a gate of the second transistor, and a drain of thethird transistor is connected to the source of the first transistor. 6.The current source of claim 4, further comprising a fourth transistor,the fourth transistor cascoding the second transistor forming asymmetrical voltage-balanced current mirror.
 7. The current source ofclaim 1, wherein the op-amp comprises an operational transconductanceamplifier (OTA).
 8. The current source of claim 1, wherein the source ofthe first transistor is connected to the supply and the drain of thefirst transistor is the output terminal of the current source, andwherein a gate of the second transistor is connected to an output of theop-amp, a source of the second transistor is connected to the supply,and a drain of the second transistor is connected to the input of theop-amp.
 9. The current source of claim 1, wherein the first transistorand the second transistor are NMOS transistors.
 10. The current sourceof claim 1, wherein the first transistor and the second transistor arePMOS transistors.
 11. The current source of claim 1, wherein a drain ofthe second transistor is connected to the input of the op-amp, andwherein responsive to a change in the supply voltage, an output voltageat an output of the op-amp drives the first transistor to maintain thesubstantially constant current.
 12. A current source, comprising: anoperational amplifier (op-amp); a control voltage source connected to afirst input of the op-amp and a ground reference node; a feedbackresistor connected to a second input of the op-amp and the groundreference node; a first transistor, wherein a gate of the firsttransistor is connected to an output of the op-amp and a source of thefirst transistor is connected directly or through a current mirror to avoltage supply, wherein a drain of the first transistor is an outputterminal of the current source; a second transistor configured tofeed-back supply noise to the feedback resistor, wherein responsive tothe second transistor feeding back the supply noise to the feedbackresistor, the op-amp outputs the supply noise to the gate of the firsttransistor; a capacitor connected to the ground reference node; and aresistor connected to the capacitor and the output of the op-amp,wherein the resistor and the capacitor define a noise bandwidth of thecurrent source.
 13. The current source of claim 12, further comprising:a second resistor, the second resistor connected between the source ofthe first transistor and the voltage supply such that the source of thefirst transistor is connected to the voltage supply through the secondresistor directly or through the current mirror; and a third resistor,the third resistor connected between the source of the second transistorand the voltage supply such that the source of the second transistor isconnected to the voltage supply through the third resistor.
 14. Thecurrent source of claim 12, wherein the current source outputs thesubstantially constant current based upon a resistance value of thefeedback resistor and a voltage output of the control voltage source.15. The current source of claim 12, further comprising a thirdtransistor, the third transistor forming the current mirror with thesecond transistor, the current mirror configured to increase an outputimpedance of the current source at the output terminal of the currentsource.
 16. The current source of claim 15, further comprising a fourthtransistor, the fourth transistor cascoding the second transistorforming a symmetrical voltage-balanced current mirror.
 17. A currentsource configured to output a substantially constant current,comprising: an operational amplifier (op-amp); a feedback resistorconnected to an input of the op-amp and a ground reference node; a firsttransistor that sources current from a supply, wherein a drain of thefirst transistor is an output terminal of the current source; and asecond transistor configured to feed-back supply noise to the feedbackresistor, wherein responsive to the second transistor feeding back thesupply noise to the feedback resistor, the op-amp outputs the supplynoise to a gate of the first transistor; wherein a source of the firsttransistor is connected directly or through a current mirror to thesupply, and wherein a source of the second transistor is connecteddirectly or through a second resistor to the supply.
 18. The currentsource of claim 17, further comprising a third transistor, the thirdtransistor forming the current mirror with the second transistor, thecurrent mirror configured to increase an output impedance of the currentsource at the output terminal of the current source.
 19. The currentsource of claim 18, further comprising a fourth transistor, the fourthtransistor cascoding the second transistor forming a symmetricalvoltage-balanced current mirror.
 20. A current source, comprising: anoperational amplifier (op-amp); a control voltage source connected to afirst input of the op-amp and a ground reference node; a feedbackresistor connected to a second input of the op-amp and the groundreference node; a first transistor, wherein a gate of the firsttransistor is connected to an output of the op-amp and a source of thefirst transistor is connected directly or through a current mirror to avoltage supply, wherein a drain of the first transistor is an outputterminal of the current source; a second transistor configured tofeed-back supply noise to the feedback resistor, wherein responsive tothe second transistor feeding back the supply noise to the feedbackresistor, the op-amp outputs the supply noise to the gate of the firsttransistor; a second resistor, the second resistor connected between thesource of the first transistor and the voltage supply such that thesource of the first transistor is connected directly or through thecurrent mirror to the voltage supply through the second resistor; and athird resistor, the third resistor connected between the source of thesecond transistor and the voltage supply such that the source of thesecond transistor is connected to the voltage supply through the thirdresistor.